Atomic Number, Mass Number & Isotopes (WJEC GCSE Chemistry: Combined Science)

Atomic Number

• The atomic number (or proton number) is the number of protons in the nucleus of an atom
  • The symbol for this number is Z

• It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table
• The proton number is unique to each element, so no two elements have the same number of protons
• Electrons can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction

Mass Number 

• The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom
  • The symbol for this number is A

• The mass number minus the proton number gives you the number of neutrons of an atom
• Note that protons and neutrons can collectively be called nucleons
• The atomic number and mass number for every element is on the periodic table

Atomic Number & Mass Number diagram

Diagram showing the notation used on the periodic table

Isotopes

• Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number
• So, C-14 is the isotope of carbon which contains 6 protons and 6 electrons, but the 14 signifies that it has 8 neutrons (14 - 6 = 8)
  • It can also be written as ¹⁴C

• Isotopes display the same chemical characteristics
• This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry
• The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only

Table to show the structures of isotopes of hydrogen

Isotope	Atomic Structure	Symbol
Hydrogen - 1
Hydrogen - 2
Hydrogen - 3

• The atomic number is equal to the number of protons (p) in an atom
• Since atoms are neutral, then it is also the same as the number of electrons (e)
• The mass number is the number of protons plus neutrons
• The number of neutrons (n) can thus be calculated by subtracting the atomic number from the mass number
• For example, beryllium has an atomic number of 4, therefore it has 4 protons and 4 electrons
• The mass number of beryllium is 9, so it has 9 - 4 = 5 neutrons
• The PEN numbers for beryllium are thus:
  • p = 4
  • e = 4
  • n = (9 - 4 =) 5

Diagram showing an element from the Periodic Table

The symbol key for beryllium as represented on the Periodic Table